1. Field of the Invention
The present invention relates to cast iron gears, such as those used as cam gears on engines and as used for fuel injection gear pumps. Still further, the invention is concerned with a process by which such iron gears can be cast, especially in a vertical orientation.
2. Background of the Invention
Ever since the earliest attempts to produce cast iron and steel wheels and gears, problems have been encountered due to the fact that such products have a hub and a rim that are thick in comparison to the relatively thin web used to interconnect the hub with the rim. That is, this type of product does not cool uniformly, and due to the thin nature of the web, it solidifies before the hub and rim, and this solidification of the web acts as a dam that cuts off a continuing supply of molten metal between the hub and rim, thereby producing areas of porosity due to the inability to fully compensate for shrinkage of the metal as it continues to cool.
To correct for the above-noted problem, the primary focus, from the beginning to now, has been on attempts to modify the normal cooling pattern so as to increase the rate at which the metal in the rim and hub cools while slowing the rate at which the web cools. For example, in U.S. Pat. No. 790,202, thermally conductive chill elements are placed so as to come into contact with the periphery of the rim and the axial ends of the hub, while nonconductive material, such as sand, is in contact with the surfaces of the web. Likewise, in U.S. Pat. No. 2,420,003, a mold is disclosed that is designed to selectively cool the parts of a cast wheel so that the relatively thin web will not prematurely solidify cutting off shrinkage compensating flow between the thick hub and rim of the wheel.
However, the use of chill elements not only makes the mold used more expensive, but it increases the cost of producing the cast product and is not well suited to modern mass production techniques. Likewise, the use of mold with selective cooling arrangements only makes the molds more complicated and expensive.
Furthermore, when a gear or wheel having a thick rim and hub connected by a thin web are produced in a vertical orientation (as is the case in high volume, production lines using "DISAMATIC" type machines; see FIGS. 4 and 5 and 2130 DISAMATIC MOLDING SYSTEM, TECHNICAL DESCRIPTION, Edition 190184), problems of shrinkage cavity porosity regions being formed are compounded by the fact that it is not feasible, at least from an economic standpoint, to feed molten metal to the mold at both the hub and rim. Gears produced in a vertical orientation with molten iron fed at the top of the rim have been found to have areas of extensive porosity in the hub. In initial failed attempts by the present applicants to solve this porosity problem in the vertical production of gears, a symmetrical array of three fed ribs were formed on the web and windows were provided between each pair of feed ribs to compensate for the increased weight of the ribs. Such an approach was found to produce areas of porosity in the rim, especially in the vicinity of the lower ribs at the bottom of the gear, without fully correcting the porosity problem in the hub.